1. Field of the Invention
The present invention relates to a water-resistant telecommunication cable, in particular an optical fibre cable, comprising a solid and compact element, in particular a buffer tube, housing a loose transmitting element.
The present invention also relates to a method for maintaining loose the transmitting element of the cable of the invention upon the extrusion thereof and to the use of a water-soluble polymer material, comprising a vinyl alcohol/vinyl acetate (referred to hereinbelow as VA-VAc copolymer and generally identified in the art as PVA), for the preparation of the solid and compact element in order to maintain loose the transmitting element upon extrusion of the cable.
2. Description of the Related Art
In extruding polyvinyl alcohol (PVA) several problems have been generally encountered. It is known that the melting point of PVA is higher than its decomposition temperature and that the risk of cross-linking is high. The extrudability of PVA has hitherto been sought to be improved, e.g., by the addition of plasticizers, commonly a polyhydric alcohol and lubricants such as stearic acid; the result being however of limited utility because the use of PVA requires to work within a very narrow temperature range. Also the fugitive nature of the plasticizer could generate a tendency to stick thereby interfering with a smooth extrusion. The kind and amount of the plasticizer are also important to get a composition neither too tacky in use and nor too soft for the manufacturing process.
U.S. Pat. No. 3,607,812 discloses a method of manufacturing a PVA film insoluble in water at a temperature below 40° C., by adding 5–13 parts by weight of a polyhydric alcohol plasticizer such as glycerine, triethylene glycol, triethylol propane, to 87–95 parts by weight of a fully hydrolysed PVA; the film is useful for making hospital bags or the packaging material of detergents and agricultural chemicals.
U.S. Pat. No. 3,997,489 discloses a PVA composition of improved melt flow characteristics, the PVA having a degree of polymerisation of about 500–2,000 and a degree of hydrolysis of at least about 88%; the composition comprises 0–20% of a plasticizer, about 0.5–5% of a wax and about 0.5–5% of an ethylene polymer, all amounts being based upon the weight of the PVA-plasticizer blend. The optional presence of a plasticizer is said to enhance the improvement of the melt flow characteristics.
GB 2,340,835 discloses an extrudable PVA composition comprising a blend of partially and fully hydrolysed PVAs, a lubricant such as a fatty acid amide and, optionally, up to 20 wt % of a plasticizer such as glycerine, ethylene glycol, etc. The composition is said to be suitably extruded on current unmodified extrusion lines without processing problems such as thermal degradation and high temperature cross-linking.
U.S. Pat. Nos. 4,323,492 and 4,542,178 disclose a tack-free granular PVA composition, and a process for the manufacture thereof, capable of being processed thermoplastically. The composition comprises a 10–30 wt % of a plasticizer, f.i. glycerol, polyethylene glycol, trimethylol propane, an amount of water insufficient to dissolve the PVA granules, and 2–12 wt % of a fine particle high molecular weight organic compound, f.i. starch, cellulose, casein, amounts being referred to the unplasticized PVA granules.
U.S. Pat. No. 4,611,019 discloses a melt extrudable composition consisting essentially of a mixture of a PVA homopolymer, having a degree of hydrolysis greater than 95% and a molecular weight range which spans the solution viscosity range of 2 to 30 cP at 4% concentration in water at 20° C., and 7–15 wt % of a plasticizer such as aromatic sulphonamides, polyols, etc., and 0.5–4.5 wt % of a polyamide, amounts being based on the PVA. The composition can be extruded and is particularly useful, as a film, for protecting oxygen sensitive products.
EP-A-0635545 discloses a melt extrudable PVA composition, especially useful for injection moulding of articles, in particular personal care articles which are flushable and biodegradable. The composition consists essentially of a blend of partially (30–50%) and fully hydrolysed (50–70%) PVAs, a solid (1–10%) and a liquid (8–20%) plasticizer, the amount of the plasticizers being referred to PVA. The PVA blend is said to be used because only fully hydrolysed PVA would give the moulded articles an excessive brittleness which would be offset by using an excess of plasticizer which, in its turn, would lead to lower water resistance; on the other hand, using an all partially hydrolysed PVA composition would result to be too tacky in use and too soft for the manufacturing process. Preferred plasticizers are polyhydric alcohols, the preferred solid ones being sorbitol, mannitol, pentaerythritol or dipentaerythritol and the preferred liquid ones being glycerol, ethylene glycol and polyethylene glycol. Further, an exudation of the liquid plasticizer on the surface of the article, and upon ageing, especially with fully hydrolysed PVA, is noted at greater than 10 wt % solid plasticizer and low levels of liquid plasticizer.
In order to prevent a PVA powder from originating coarse lumps of a degraded and partially cross-linked material upon extrusion, U.S. Pat. No. 4,469,837 discloses a thermoplastic extrudable PVA composition comprising a dry mixture of PVA, preferably having a hydrolysis degree greater than 70% and a degree of polymerisation of 500–2500, with one or more solid monomeric polyhydroxylated alcohol which, as such or in mutual admixture, exhibit the main melting point peak ranging from 160° to 230° C., such as pentaerythritol, bipentaerythritol and trimethylolethane, preferably in an amount of 10–50 parts per 100 parts of the PVA. The addition of small percentages of other liquid alcohols, such as glycerols or glycols, is further contemplated so to maintain the melting point of the polyhydroxylated alcohol(s) within the above mentioned temperature range.
EP-A-0860471 discloses a three stages process for the preparation of a mouldable and extrudable solid thermoplastic composition comprising partially or totally hydrolysed PVA, 1–10 parts of one or more solid plasticizers, 5 to 30 parts of a liquid mixture containing one or more hydroxylated organic compound, water and at least a salt of an alkaline or alkaline earth metal, 15–30 parts of a solid mixture containing solid polyhydroxylated alcohols, glycols and glycolic ethers, liquid alkanolamines, mineral or organic acids, hydrated inorganic salts stabilizers, other compatible polymers and/or copolymers having low molecular weight and possible mineral charges. The resulting composition is reported to show a good plasticization of the PVA in the transformation equipments and a good workability constancy in time and results to be transformable in manufactures provided with good flexibility. The composition obtained by the disclosed process is useful for the production of granulates, flat or tubular films, moulded articles of manufacture, plates or films coextruded with polyolefins, polyvinyl chloride, polystyrene or polyamides.
WO00/21098 discloses a water-resistant cable, particularly an optical fibre cable, comprising a longitudinal cavity extending along the length of the cable, at least one optical fibre housed inside the cavity and a solid and compact element associated with the cavity, said element being made of a water-soluble material which, upon contact with water, dissolves itself, at least partially, and forms a viscous solution capable of stopping the longitudinal flow of water along such element. Solid and compact elements of this kind allow to avoid the use, or at least substantially reduce the amount, of conventional water-blocking means, such as grease-like material, water-swellable powders and the like. Said solid and compact element is, particularly, a buffer tube, preferably made of a VA-VAc copolymer having a hydrolysis degree of from about 50% to 95%, preferably from 70% to about 90%. This copolymer, in the presence of relative humidity percentages of less than 75–80%, does not show any phenomena of surface stickiness caused by the partial absorption of water. The use of conventional plasticizers, in an amount of at least 5% of the total weight of the polymer material composition, is further disclosed.
The addition of plasticizers to the polymer material, generally in an amount of about 1% to about 30% of the weight of the latter, is well known, in the field, in order to improve the processability and final flexibility of the material; it is further known that the plasticizers are also capable of increasing the ability of the water-soluble polymer material to absorb water.
EP01130960 discloses a water-resistant telecommunication cable, particularly an optical fibre cable comprising an elongated element housing at least one transmitting element comprising a water-soluble polymer composition which comprises a VA-VAc copolymer having a hydrolysis degree of from about 60% to about 95%, a plasticizer, and a hydrolysis stabiliser comprising a chelant. The presence of the stabiliser allows to reduce the increase of the hydrolysis degree of the VA-VAc copolymer upon ageing, thus maintaining the desired water-blocking properties of the VA-VAc copolymer. Further, there is disclosed the addition of a plasticizer such as glycerol, sorbitol, trimethylolpropane, low molecular weight polyglycol, such as polyethylene glycol (e.g. di- or tri-ethyleneglycol), pentaerythritol, neopentylglycol, triethanolamine or oxyethylated phosphoric esters, preferably in an amount of from about 5% to about 30% by weight with respect to the weight of VA-VAc copolymer.
The Applicant has now observed that while buffer tubes, as disclosed in WO00/21098, comprising a partially hydrolysed VA-VAc copolymer allow to avoid the use, or at least substantially reduce the amount, of conventional water-blocking means and that while adding a stabiliser, as disclosed in EP01130960, allows to reduce the increase of the hydrolysis degree of the VA-VAc copolymer upon ageing, thus maintaining the desired water-blocking properties of the VA-VAc copolymer, a stickiness of said copolymer is experienced during the estrusion process generating an interference with the transmitting element, f.i. optical fibres, put inside the solid and compact element, such as a buffer tube, of the cable.
Particularly, the Applicant noted that the water-soluble VA-VAc based polymer material resulted to be sticky during its extrusion, regardless of the presence of a stabiliser and a plasticizer and of the fact that the water-soluble polymer material is extruded alone or in combination with the water-insoluble polymer materials commonly used for the outer layer of a solid and compact element, such as a the buffer tube, of a transmission cable.
The Applicant observed that, during the extrusion of, f.i., a buffer tube for an optical fibre cable, the stickiness of the water-soluble polymer material, due to the prevailing of the viscous component of the melt, generates a negative interaction with the surface of the optical fibres, this being detectable both co-extruding different polymer materials, which is industrially preferred, and even singly extruding them, although the problem is less evident in the latter case.
In fact, in both cases, the Applicant experienced that sticking phenomena affect the quality of the resulting transmission cable regardless of possible adjusting the common industrial extrusion process by, f.i. slowing the production.
The Applicant has also noted that, extruding the PVA composition disclosed in U.S. Pat. No. 3,997,489, the polyethylene waxes separate from PVA, due to their mutually different polarity, thus slowing the interaction with the water possibly penetrated in the cable and actually making this composition useless as a water-blocking mean aid in a water-resistant communication cable. The same shortcoming has been also experienced by the Applicant with the composition disclosed in GB 2,340,835 where the fatty acid amine separates upon extrusion from PVA, because—once again—of their different polarity.
The Applicant has also observed that using release agents such as oils or powders on, f.i., optical fibres to prevent the above described stickiness, results to be unsuitable to guarantee a regular extrusion process, also because it is cumbersome to uniformly distribute the release agent on the fibres; besides, oils and/or powders move along the optical fibres distributing themselves irregularly this, especially with powders, possibly bringing to a mechanical interference between the tube and fibres.
Further, the solid and compact element, f.i. a buffer tube, housing the transmission element, f.i. an optical fibre, has to be dimensionally stable and regular in order to avoid any interference of the optical fibres with the polymer material comprised in the solid and compact element, during the extrusion step.
The fibers and the plastic material tube enclosing them, proceed for a certain length along the extrusion line, independently from one another. The dimensional stability of the molten state of the water-soluble polymer material and the stickiness phenomena to optical fibres can be observed between the extrusion head and the final collection reel.
When the stickiness phenomena occur, the water-soluble polymer material and optical fibres proceed together and during the coooling step the shrinkage of the water-soluble polymer material cause the macrobending of the optical fibres.
The macrobending gives rise to attenuation phenomena of the transmitted signal, on account of either irregularly distributed pressures on the surface of the transmission element or excessive ringing of the transmission element, both of which being drawbacks which can result in attenuation phenomena of the transmitted signal, even under conditions which would otherwise not be harmful to the functioning of the cable.
Specifically, the stickiness of the water-soluble VA-VAc based polymer material results in potentially damaging the transmission element and, at the same time, does not guarantee the desired and needed stability and regularity upon its extrusion, this further potentially giving rise to attenuation phenomena of the transmitted signal.